Firewalls.

Firewalls. Intruders often attempt to gain
access to networked systems by pretending to initiate connections from trusted
hosts. They squash the emissions of the genuine host using a denial-of-service
attack and then attempt to connect to a target system using the address of the
genuine host. To counter these address-spoofing attacks and enforce limitations
on authorized connections into the organizationĚs network, it is necessary to
filter all incoming and outgoing network traffic.

A firewall is a collection of hardware and software designed to examine a
stream of network traffic and service requests. Its purpose is to eliminate from
the stream those packets or requests that fail to meet the security criteria
established by the organization. A simple firewall may consist of a filtering
router, configured to discard packets that arrive from unauthorized addresses or
that represent attempts to connect to unauthorized service ports. More
sophisticated implementations may include bastion hosts, on which proxy
mechanisms operate on behalf of services. These mechanisms authenticate
requests, verify their form and content, and relay approved service requests to
the appropriate service hosts. Because firewalls are typically the first line of
defense against intruders, their configuration must be carefully implemented and
tested before connections are established between internal networks and the
Internet.